1. Field of the Invention
The present invention relates generally to wireless communication systems, and more specifically, to a method and apparatus for improving the probability of acquiring service in a xe2x80x9cborder areaxe2x80x9d between two service areas.
2. Related Art
A cellular communications system typically provides services to a geographic service area that includes many smaller geographic areas, known as cells. Each cell is serviced by a transmitter-receiver station, also known as a cell site or base station. The cell sites are connected through land lines, or other communication links, such as microwave links, to a mobile telephone switching office (MTSO). The MTSO is connected to the public switched telephone network.
Under current FCC rules, any given geographic service area may be serviced by up to two competing providers of cellular airtime communications services (also referred to as service providers, system providers, cellular carriers or simply xe2x80x9ccarriersxe2x80x9d). Each of the two carriers in any given geographic location, which are commonly referred to as xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d carriers, is associated with a system identification code (SID) that uniquely identifies the carrier. For example, carrier A may be Bell Atlantic and may have a SID=46, and carrier B may be Cellular One and have a SID=25. The two service providers in any given geographic service area are assigned different groups (or sets) of frequencies through which services are provided. That is, the A carrier is assigned a first set of frequencies and the B carrier is assigned a second set of frequencies that do not overlap the A set of frequencies.
Each given geographic service area typically has one or more neighboring geographic service areas. Each neighboring geographic service area is also serviced by two carriers, which shall be referred to as xe2x80x9cAxe2x80x2xe2x80x9d and xe2x80x9cBxe2x80x2xe2x80x9d carriers. The A carrier and the Axe2x80x2 each have a different SID. Similarly the B carrier and the Bxe2x80x2 carrier each have a different SID. However, the A carrier and the Axe2x80x2 carrier both provide service using the same set of frequencies. Similarly, the B carrier and the Bxe2x80x2 carrier both provide service using the same set of frequencies. The area where two geographic service areas meet (for example, where a cell of carrier A overlaps a cell of carrier Axe2x80x2) is often referred to as a xe2x80x9cborder areaxe2x80x9d. As will be described below, a user terminal may experience problems acquiring service within such border areas. Accordingly, an object of the present invention is to improve system acquisition in these xe2x80x9cborder areas.xe2x80x9d
The present invention is directed toward a method and apparatus for acquiring service in a border area that is serviced by two communications systems that share the same frequencies. The method of the present invention includes the steps of scanning a predetermined range of control channels in order to determine the first and second strongest control channels. A determination is then made as to whether a first system identification code (SID) associated with a signal received over the strongest control channel is acceptable. This can be accomplished, for example, by comparing the first SID to a list of acceptable SIDs. Conventionally, if the SID associated with the signal received over the strongest control channel is not acceptable, the system acquisition process will be aborted. However, in the present invention, the probability of acquiring service is increased because a determination is made of whether a SID associated with the signal received over the second strongest control channel is acceptable, if the first SID was found not to be acceptable.
Conventionally, the SID associated with the signal received over the second strongest control channel is only determined if the signal received over the strongest control channel cannot be successfully demodulated. For example, this can occur if the signal received over the strongest control channel is a signal made up of noise rather than a control signal. In contrast, in the method of the present invention, the SID of the second strongest channel is considered even if the signal received over the strongest channel was successfully demodulated (associated with an unacceptable SID). This inventive method significantly increases the probability that a user terminal will acquire service in an area where the strongest control signal received by a user terminal is associated with an unacceptable SID (in other words, associated with an unacceptable carrier).
The method of the present invention further comprises the steps of saving the first SID, if the first SID is acceptable. If the first SID is found not to be acceptable, the second SID is saved if the second SID is found to be acceptable.
Paging channels are then scanned and the strongest and second strongest paging channels are determined. Conventionally, if the signal received over the strongest paging channel is actually a paging signal, system acquisition will be aborted if the SID associated with the strongest paging channel is not equal to the saved SID. The present invention increases the probability that the user terminal (UT) will acquire service by determining whether the SID associated with the second strongest paging channel is equal to the saved SID, if the SID associated with the strongest channel was not equal to the saved SID.
Conventionally, the SID associated with the signal received over the second strongest paging channel is only determined if the signal received over the strongest paging channel was not actually a paging signal. For example, this can occur if the signal received over the strongest paging channel is a signal made up of noise rather than a paging signal. In contrast, in the method of the present invention, the SID of the second strongest paging channel is considered even if the signal received over the strongest paging channel was actually a paging signal (e.g., the strongest paging channel was associated with a SID which was not equal to the saved SID).
The probability of acquiring service can be even further increased by having the UT look beyond the second strongest channels (control and/or paging). This and other features of the present invention are disclosed in more detail below.